Does texas law state that u have 72 hrs to cancel an auto contract with a dealer?
No. Once you have taken delivery of a vehicle, you can not cancel the contract. The 72 hour rule that you are thinking of applies to sales made in your home. It is designed to prevent you from being pressured to buy in your own home. In a place of business you have the option of leaving at any time. You can not walk out of your own home to get away from a high pressure sales pitch.
